An inquest jury has found neglect contributed to the death of a mental health blogger while she was being cared for at a psychiatric unit.
Beth Matthews, 26, died a short time after taking the substance which she had ordered online from Russia.
She told staff at Priory Hospital Cheadle Royal in Stockport the toxic material was protein powder.
A nine-day inquest at Manchester South Coroners’ Court concluded she died from suicide contributed to by neglect on March 21 last year.
The hearing was told Ms Matthews, originally from the village of Menheniot near Liskeard, Cornwall, was being detained under Section 3 of the Mental Health Act for a personality disorder.
She was a complex patient considered at high risk of taking her life and had a history of frequent attempts.
Ms Matthews’s care plan stated she should not be allowed to open her own mail.
Following the conclusion, her family - including mother Jane Matthews - said she had been “let down” by the Priory, adding her death was “wholly avoidable” and “completely unnecessary”.
“Mental health care providers must listen to and act on the findings of this inquest,” they added.
“It is incumbent on them to keep their patients safe.”
Paying tribute, the family said: “Beth tried to help others through describing her own mental health experiences in a highly graphic but articulate way and by doing so was able to touch and help countless others.
“We know for a fact that she saved at least one person through her social media presence.
“That is a huge legacy for a young lady to leave behind.”
The inquest jury found that while under the care of the Priory “it was evident there were serious inconsistencies across all levels of her care plan".
The hospital provided “inadequate care of a highly vulnerable patient,” they said, and jurors found there was a widespread “lack of communication, failing to escalate serious risk factors, lack of team cohesion, and reliance on inaccurate and inadequate information”.
The court heard weeks before her death, she told a member of staff she could purchase something “to do the job”.
Another care worker wrote in her notes that everything Ms Matthews received “must be checked”, and stated “we need to be opening her parcels for her”.
But the court heard she was allowed to open the package, which she had ordered from Russia, with two staff monitoring her at arm’s length.
The jury found the supervising staff were “unable to prevent her from consuming” the substance and the evidence demonstrated there had been a “frequent deviation” from her care plan.
The Priory Group admitted Ms Matthews’s plan had not been followed and if it had, she would not have been able to ingest the substance.
The mental health blogger, who was described in court as “vivacious” and “bright”, had tens of thousands of social media followers.
Psychiatrist Dr Alind Srivastava, who works for the Cornwall Partnership NHS Foundation Trust, previously told the jury she “did a lot of good on social media in helping other people, in explaining what happened to her”.
A spokesman for the Priory Group said: “We fully accept the jury’s findings and acknowledge that far greater attention should have been given to Beth’s care plan.
“At the time of Beth’s unexpected death, we took immediate steps to address the issues around how we document risk and communicate patients’ care plans, alongside our processes for receiving and opening post.
“We want to extend our deepest condolences to Beth’s family and friends for their loss. Beth’s attempts to overcome her mental health challenges had been an inspiration for many.
“Although unexpected deaths are extremely rare, we recognise that every loss of life in our care is a tragedy.”
Speaking after the jury delivered their conclusions, Assistant Coroner Andrew Bridgman offered his condolences to Ms Matthews’ family.
He said: “There are no words to express for the loss of your daughter. The loss of a child is something no person should have to suffer.”
Clinical negligence expert Stephen Jones, the family’s solicitor at Leigh Day, added: “This was a particularly upsetting inquest.
“Beth’s death came about because a very simple and straightforward instruction in her care plan, that staff should open parcels for her, was not followed.
“Had the care plan been followed, Beth would not have died. We hope that the jury’s finding that Beth’s death was contributed to by neglect will help shine a light on what happened and emphasise the need for improvements to be made.”